Conventional magnetic-resonance (MR) machines (such as MR imaging (MRI) machines or MR spectroscopy (MRS) machines) employ a high-field-strength magnet (e.g., a superconducting-coil magnet having a constant (DC) magnetic field of about one tesla (1 T) or more, as well as a gradient magnet coil (which generates an additional magnetic field whose strength varies over space in a desired manner), and a radio-frequency (RF) transmit-and-receive coil device (RF-coil device) that includes a plurality of transmit-antenna elements and a plurality of receive-antenna elements. RF signal generators and RF power amplifiers are conventionally located in a control room remote from the RF-coil devices, and the RF power amplifiers have their output signals combined and then coupled to the RF-coil device via high-power-capability well-shielded coaxial cables (frequently called just “coax”). Such conventional designs typically use a plurality of medium-high power amplifiers (e.g., in some conventional circuits, each such medium-high power amplifiers is capable of outputting 500 W to 5000 W) together to amplify an RF signal, thus generating a plurality of medium-high-power RF signals (e.g., wherein the total RF power of this plurality of signals may be as high as 50 kW), and these are combined (using a combiner circuit that often incurs losses of up to 50% or more of the signal) to form a single very-high-power signal (of perhaps 30 kW, due to losses taken from the 50 kW in the plurality of medium-power signals) and then transmitted (using coaxial cables that typically incur additional losses of up to 50% or more of the signal by the time the signal is coupled to the RF-coil device in the MR magnet bore (the opening through the center of the DC magnet)). Because of the high power of the RF transmit signal, it is infeasible to wirelessly couple it from the remote control-room power amplifiers to the RF-coil device in the MR magnet bore.
U.S. patent application Ser. No. 12/719,841 titled “REMOTELY ADJUSTABLE REACTIVE AND RESISTIVE ELECTRICAL ELEMENTS AND METHOD” filed 8 Mar. 2010 by Carl Snyder et al. is incorporated herein by reference. Snyder et al. describe an apparatus and method that include providing a variable-parameter electrical component in a high-field environment and based on an electrical signal, automatically moving a movable portion of the electrical component in relation to another portion of the electrical component to vary at least one of its parameters. In some embodiments, the moving uses a mechanical movement device (e.g., a linear positioner, rotary motor, or pump). In some embodiments of the method, the electrical component has a variable inductance, capacitance, and/or resistance. Some embodiments include using a computer that controls the moving of the movable portion of the electrical component in order to vary an electrical parameter of the electrical component. Some embodiments include using a feedback signal to provide feedback control in order to adjust and/or maintain the electrical parameter. Some embodiments include a non-magnetic positioner connected to an electrical component configured to have its RLC parameters varied by the positioner.
The basis of MRI is the directional magnetic field, or moment, associated with charged particles in motion. Nuclei containing an odd number of protons and/or neutrons have a characteristic motion or precession. Because nuclei are charged particles, this precession produces a small magnetic moment. When a human body is placed in a large magnetic field, many of the free hydrogen nuclei align themselves with the direction of the magnetic field. The nuclei precess about the magnetic field direction like gyroscopes. This behavior is termed Larmor precession. The frequency of Larmor precession is proportional to the applied magnetic field strength as defined by the Larmor frequency, ω0=γB0, where γ is the gyromagnetic ratio and Bo is the strength of the applied magnetic field. The gyromagnetic ratio is a nuclei specific constant. for hydrogen, γ=42.5 MHz/Tesla. To obtain an MR image of an object, the object is placed in a uniform high-strength magnetic field, of between 0.5 to 1.5 Tesla. As a result, the object's hydrogen nuclei align with the magnetic field B0 and create a net magnetic moment M0 parallel to B0. Next, a radio-frequency (RF) pulse, Brf, is applied perpendicular to B0. This pulse, with a frequency equal to the Larmor frequency, causes M to tilt away from B0. Once the RF signal is removed, the nuclei realign themselves such that their net magnetic moment, M, is again parallel with B0. This return to equilibrium is referred to as relaxation. During relaxation, the nuclei lose energy by emitting their own RF signal. This signal is referred to as the free-induction decay (FID) response signal. The FID response signal is measured by a conductive field coil placed around the object being imaged. This measurement is processed or reconstructed to obtain 3D MR images. (This paragraph is by Blair Mackiewich, Masters thesis, 1995)
for example, approximately 64 MHz is used for MRI machines having 1.5-Tesla magnets (these are used for most MR machine platforms in the world today), 128 MHz is used for MRI machines having 3-T magnets (currently the fastest-growing segment of the MR market), 300 MHz is used for MRI machines having 7-T magnets (considered the highest-field machines supported by industry today), 400 MHz is used for MRI machines having 9.4-T magnets (it is believed there are now only three in use in the world), and 450 MHz is used for the MRI machine having a 10.5-T magnet (currently, just the Center for Magnetic Resonance Research (CMRR) at the University of Minnesota operates one of these).
U.S. Pat. No. 4,682,125 to Harrison et al. issued Jul. 21, 1987 titled “RF coil coupling for MRI with tuned RF rejection circuit using coax shield choke” is incorporated herein by reference in its entirety for all purposes. Harrison et al. describe undesirable RF coupling via the outside of an outer coaxial cable conductor to/from RF coils in a magnetic resonance imaging apparatus is minimized by employing a parallel resonance tuned RF choke in the circuit. The choke is realized by forming a short coiled section of the coaxial cable with a lumped fixed capacitance connected in parallel thereacross and a conductive tuning rod positioned within the center of the coiled section so as to trim the parallel resonant frequency to the desired value.
U.S. Pat. No. 4,763,076 to Arakawa et al. issued Aug. 9, 1988 titled “MRI transmit coil disable switching via RF in/out cable” is incorporated herein by reference in its entirety for all purposes, and describes a detuning/decoupling arrangement for a Magnetic Resonance Imaging (MRI) system RF coil arrangement (of the typing using the nuclear magnetic resonance (NMR) phenomenon) that uses switching diodes to selectively connect and disconnect portions of an RF resonant circuit in response to a DC control signal. The DC control signal selectively forward biases and reverse biases the switching diodes. The DC control current is fed to the resonant circuit along the same RF transmission line used to feed RF signals to/from the circuit. An in-line coaxial shielded RF choke connected to the RF transmission line isolates the DC control signals from the RF signals flowing on the same transmission line—reducing the number and complexity of isolation devices required on the ends of the transmission line to separate the RF and DC signals.
Conventional MR machines and their components and operation are described in numerous patents and patent application such as U.S. Pat. No. 4,947,119 to Ugurbil et al., U.S. Pat. No. 5,908,386 to Ugurbil et al., U.S. Pat. No. 6,650,116 to Garwood et al., U.S. Pat. No. 6,788,056 to Vaughan et al., U.S. Pat. No. 6,788,057 to Petropoulos et al., U.S. Pat. No. 6,788,058 to Petropoulos et al., U.S. Pat. No. 6,930,480 to Fijita et al., U.S. Pat. No. 6,946,840 to Zou et al., U.S. Pat. No. 6,958,607 to Vaughan et al., U.S. Pat. No. 6,969,992 to Vaughan et al., U.S. Pat. No. 6,975,115 to Fujita et al., U.S. Pat. No. 6,977,502 to Hertz, U.S. Pat. No. 6,980,002 to Petropoulos et al., U.S. Pat. No. 7,042,222 to Zheng et al., U.S. Pat. No. 7,084,631 to Qu et al., U.S. Pat. No. 7,279,899 to Michaeli et al., U.S. Pat. No. 7,403,006 to Garwood et al., U.S. Pat. No. 7,514,926 to Vaughan et al., U.S. Pat. No. 7,598,739 to Vaughan et al., U.S. Pat. No. 7,633,293 to Olson et al., U.S. Pat. No. 7,710,117 to Vaughan et al., U.S. Patent Publication 2004 027128A1 to Vaughan et al., U.S. Patent Publication 2006 001426A1 to Vaughan et al., U.S. Patent Publication 2008 0084210A1 to Vaughan et al., U.S. Patent Publication 2008 0129298A1 Vaughan et al., U.S. Patent Publication 2009 115417A1 to Akgun et al., U.S. Patent Publication 2009 237077A1 to Vaughan et al., and U.S. Patent Publication 2009 0264733A1 to Corum et al.; all of which are incorporated herein by reference in their entirety for all purposes.
U.S. Pat. No. 4,947,119 to Ugurbil et al. (incorporated herein by reference in its entirety for all purposes) describes several magnetic resonance imaging (MRI) methods using adiabatic excitation. One method accomplishes slice selection with gradient-modulated adiabatic excitation. Another method employs slice selection with adiabatic excitation despite large variations in B1 magnitude. There is also described 1H spectroscopy using solvent suppressive adiabatic pulses.
U.S. Pat. No. 5,908,386 to Ugurbil et al. (incorporated herein by reference in its entirety for all purposes) describes contrast preparation based on Modified Driven Equilibrium fourier Transfer and generates T1 weighted images for assessment of the myocardial perfusion with contrast agent first-pass kinetics. The preparation scheme produces T1 contrast with insensitivity to arrhythmias in prospectively triggered sequential imaging.
U.S. Pat. No. 6,650,116 to Garwood et al. (incorporated herein by reference in its entirety for all purposes) describes performing MRI and NMR spectroscopy that improves the dynamic range of the received signal by using adiabatic RF pulses for spin excitation rather than for spin inversion. The preferred adiabatic RF excitation produces a spatially varying phase across the slab, and a sharp slab profile. The phase variation is divided up by a phase-encoding gradient into voxels having a phase variation that is negligible over the width of the voxel. The phase variation in the slab-select direction is, on the whole, large enough that the peak amplitude of the received signal is reduced and the signal width broadened.
U.S. Pat. No. 6,788,056 to Vaughan et al. (incorporated herein by reference in its entirety for all purposes) describes an apparatus with a radio frequency magnetic field unit to generate a desired magnetic field. In one embodiment, the radio frequency magnetic field unit includes a first aperture that is substantially unobstructed and a second aperture contiguous to the first aperture. In an alternative embodiment, the radio frequency magnetic field unit includes a first side aperture, a second side aperture and one or more end apertures. In one embodiment, a current element is removed from a radio frequency magnetic field unit to form a magnetic field unit having an aperture. In an alternative embodiment, two current elements located opposite from one another in a radio frequency magnetic field unit are removed to form a magnetic filed unit having a first side aperture and a second side aperture.
U.S. Pat. No. 6,788,057 to Petropoulos et al. (incorporated herein by reference in its entirety for all purposes) describes an MRI gradient coil set that includes a uniplanar Z-gradient coil, a biplanar X-gradient coil, and a biplanar Y-gradient coil. The coil set provides an open Z-axis face.
U.S. Pat. No. 6,788,058 to Petropoulos et al. (incorporated herein by reference in its entirety for all purposes) describes an MRI coil having an axis and a first end and an opposite second end with respect to said axis includes a first ring element at the first end, a second ring element, a third ring element, a fourth ring element at the second end where the first ring element encompasses a smaller area than each of the second, third, and fourth ring elements. The coil also includes a plurality of axial elements connected between the first, second, third and fourth ring elements. The third and fourth ring elements are axially closer than the first and second ring elements.
U.S. Pat. No. 6,930,480 to Fijita et al. (incorporated herein by reference in its entirety for all purposes) describes a partially parallel acquisition RF coil array for imaging a human head includes at least a first, a second and a third loop coil adapted to be arranged circumambiently about the lower portion of the head; and at least a forth, a fifth and a sixth coil adapted to be conformably arranged about the summit of the head. A partially parallel acquisition RF coil array for imaging a human head includes at least a first, a second, a third and a fourth loop coil adapted to be arranged circumambiently about the lower portion of the head; and at least a first and a second figure-eight or saddle coil adapted to be conformably arranged about the summit of the head.
U.S. Pat. No. 6,946,840 to Zou et al. (incorporated herein by reference in its entirety for all purposes) describes an MRI array coil includes a plurality of first coils in a receive coil array and a plurality of second coils in a transmit coil array. The receive coil array and the transmit coil array are electrically disjoint.
U.S. Pat. No. 6,958,607 to Vaughan et al. (incorporated herein by reference in its entirety for all purposes) describes a radio frequency magnetic field unit to generate a desired magnetic field. In one embodiment, the RF magnetic field unit includes a first aperture that is substantially unobstructed and a second aperture contiguous to the first aperture. In an alternative embodiment, the RF magnetic field unit includes a first side aperture, a second side aperture and one or more end apertures. In one embodiment, a current element is removed from a RF magnetic field unit to form a magnetic field unit having an aperture. In an alternative embodiment, two current elements located opposite from one another in a RF magnetic field unit are removed to form a magnetic field unit having a first side aperture and a second side aperture.
U.S. Pat. No. 6,969,992 to Vaughan et al. (incorporated herein by reference in its entirety for all purposes) describes an excitation and detection circuit having individually controllable elements for use with a multi-element RF coil. Characteristics of the driving signal, including, for example, the phase, amplitude, frequency and timing, from each element of the circuit is separately controllable using small signals. Negative feedback for the driving signal associated with each coil element is derived from a receiver coupled to that coil element.
U.S. Pat. No. 6,975,115 to Fujita et al. (incorporated herein by reference in its entirety for all purposes) describes a partially parallel acquisition RF coil array for imaging a sample includes at least a first, a second and a third coil adapted to be arranged circumambiently about the sample and to provide both contrast data and spatial-phase-encoding data.
U.S. Pat. No. 6,977,502 to Hertz (incorporated herein by reference in its entirety for all purposes) describes a configurable matrix receiver comprises a plurality of antennas that detect one or more signals. The antennas are coupled to a configurable matrix comprising a plurality of amplifiers, one or more switches that selectively couple the amplifiers in series fashion, and one or more analog-to-digital converters (ADCs) that convert the output signals generated by the amplifiers to digital form. for example, in one embodiment, a matrix comprises a first amplifier having a first input and a first output, and a second amplifier having a second input and a second output, a switch to couple the first output of the first amplifier to a the second input of the second amplifier, a first ADC coupled to the first output of the first amplifier, and a second ADC coupled to the second output of the second amplifier. In one embodiment, the signals detected by the antennas include magnetic resonance (MR) signals.
U.S. Pat. No. 6,980,002 to Petropoulos et al. (incorporated herein by reference in its entirety for all purposes) describes an MRI array coil for imaging a human includes a posterior array, an anterior torso array and an anterior head-neck-upper-chest array. The head-neck-upper-chest array has a head portion mountable to the anterior array and a neck-upper-chest portion hingingly attached to the head portion.
U.S. Pat. No. 7,042,222 to Zheng et al. (incorporated herein by reference in its entirety for all purposes) describes a phased-array knee coil that includes a transmit coil array and a receive coil array having a plurality of coils configured to provide a first imaging mode and a second imaging mode.
U.S. Pat. No. 7,084,631 to Qu et al. (incorporated herein by reference in its entirety for all purposes) describes an MRI array coil system and method for breast imaging. The MRI array coil system includes a top coil portion with two openings configured to receive therethrough objects to be imaged. The MRI array coil system further includes a bottom coil portion having two openings configured to access from sides of the bottom coil portion the objects to be imaged. The top coil portion and bottom coil portion each have a plurality of coil elements configured to provide parallel imaging.
U.S. Pat. No. 7,279,899 to Michaeli et al. (incorporated herein by reference in its entirety for all purposes) describes modulating transverse and longitudinal relaxation time contrast in a rotating frame based on a train of RF pulses.
U.S. Pat. No. 7,403,006 to Garwood et al. (incorporated herein by reference in its entirety for all purposes) describes magnetic resonance that uses a frequency-swept excitation wherein the acquired signal is a time domain signal is provided. In one embodiment, the sweeping frequency excitation has a duration and is configured to sequentially excite isochromats having different resonant frequencies. Acquisition of the time domain signal is done during the duration of the sweeping frequency excitation. The time domain signal is based on evolution of the isochromats.
U.S. Pat. No. 7,514,926 to Adriany et al. (incorporated herein by reference in its entirety for all purposes) describes a coil having a plurality of resonant elements and an adjustable frame. A position of at least one resonant element can be adjusted relative to at least one other resonant element. A variable impedance is coupled to adjacent resonant elements and the impedance varies as a function of a separation distance. Cables are coupled to each resonant element and are gathered at a junction in a particular manner.
U.S. Pat. No. 7,598,739 to Vaughan et al. (incorporated herein by reference in its entirety for all purposes) describes a plurality of linear current elements configured about a specimen to be imaged. A current in each current element is controlled independent of a current in other current elements to select a gradient and to provide radio frequency shimming. Each current element is driven by a separate channel of a transmitter and connected to a separate channel of a multi-channel receiver. The impedance, and therefore, the current, in each current element is controlled mechanically or electrically.
U.S. Pat. No. 7,633,293 to Olson et al. (incorporated herein by reference in its entirety for all purposes) describes technology for controlling non-uniformity in the B1 field includes selecting the phase, magnitude, frequency, time, or spatial relationship among various elements of a multi-channel excitation coil in order to control the radio frequency (RF) power emanating from the coil antenna elements. Non-uniformity can be used to steer a constructively interfering B1 field node to spatially correlate with an anatomic region of interest. A convex (quadratically constrained quadratic problem) formulation of the B1 localization problem can be used to select parameters for exciting the coil. Localization can be used in simulated Finite Difference Time Domain B1 field human-head distributions and human-head-phantom measurement.
U.S. Pat. No. 7,710,117 to Vaughan et al. (incorporated herein by reference in its entirety for all purposes) describes a current unit having two or more current paths that allows control of magnitude, phase, time, frequency and position of each of element in a radio frequency coil. for each current element, the current can be adjusted as to a phase angle, frequency and magnitude. Multiple current paths of a current unit can be used for targeting multiple spatial domains or strategic combinations of the fields generated/detected by combination of elements for targeting a single domain in magnitude, phase, time, space and frequency.
U.S. Patent Publication 2008 0129298A1 to Vaughan et al. (incorporated herein by reference in its entirety for all purposes) describes multi-channel magnetic resonance using a TEM coil.
U.S. Patent Publication 2009 0115417A1 to Akgun et al. (incorporated herein by reference in its entirety for all purposes) describes an RF having a plurality of transmission line elements, wherein at least one of the plurality of transmission line elements may have at least one dimension different than a dimension of another one of the plurality of transmission line elements. In some cases, each of the transmission line elements may include a signal line conductor and a ground plane conductor separated by a dielectric.
U.S. Patent Publication 2009 0237077A1 to Vaughan et al. (incorporated herein by reference in its entirety for all purposes) describes an RF coil system for MR applications that includes a multi-channel RF coil transceiver and a multi-channel RF coil. The RF coil system is structured for reconfiguration between a plurality of operational modes.
U.S. Patent Publication 2009 0264733A1 to Corum et al. (incorporated herein by reference in its entirety for all purposes) describes a positive contrast MRI feature using a high transverse relaxation rate contrast agent.
U.S. Pat. No. 6,495,069 issued Dec. 17, 2002 to Lussey et al. titled “Polymer composition,” is incorporated herein by reference. Lussey et al. describe a polymer composition comprises at least one substantially non-conductive polymer and at least one electrically conductive filler and in the form of granules. Their elastomer material was proposed for devices for controlling or switching electric current, to avoid or limit disadvantages such as the generation of transients and sparks which are associated with the actuation of conventional mechanical switches. They described an electrical conductor composite providing conduction when subjected to mechanical stress or electrostatic charge but electrically insulating when quiescent comprising a granular composition each granule of which comprises at least one substantially non-conductive polymer and at least one electrically conductive filler and is electrically insulating when quiescent but conductive when subjected to mechanical stress. They did not propose a means for electrically activating such switches.
U.S. Pat. No. 7,672,650 to Sorrells et al. issued Mar. 2, 2010 titled “Systems and methods of RF power transmission, modulation, and amplification, including multiple input single output (MISO) amplifier embodiments comprising harmonic control circuitry” and is incorporated herein by reference. Sorrells et al. describe methods and systems for vector combining power amplification. In one embodiment, signals are individually amplified, then summed to form a desired time-varying complex envelope signal. Phase and/or frequency characteristics of one or more of the signals are controlled to provide the desired phase, frequency, and/or amplitude characteristics of the desired time-varying complex envelope signal. In another embodiment, a time-varying complex envelope signal is decomposed into a plurality of constant envelope constituent signals. The constituent signals are amplified equally or substantially equally, and then summed to construct an amplified version of the original time-varying envelope signal. Embodiments also perform frequency up-conversion. However, neither operation in high fields nor operation where the control of the circuit is distal from the RF antennae, are discussed by Sorrells et al.
As used herein, an antenna (also called a coil element herein) is an electrically conductive elongate body that is connected to an electric circuit, and that (1) transmits (radiates) electromagnetic radiation (radio-frequency (RF) waves that propagate without a physical electrical conductor) corresponding to an alternating current (AC) radio-frequency signal from the circuit, wherein the transmitted RF waves propagate into the surrounding environment away from the coil element, and/or that (2) receives electromagnetic radiation (radio waves) from the environment and generates an AC radio-frequency electrical signal into the circuit. Coil elements can be simply a straight, bent, or coiled piece of metal wire or rod or pipe, or a similarly shaped conductor on an insulating substrate. There are a number of different types of antennae, including monopoles, dipoles, microstrips, striplines and slot antennae just to name a few, and various of these types of antennae can be formed into arrays (e.g., phased arrays) to customize the shape and strength of the resulting RF field. As used herein, an “RF-coil device” or an “antenna array” are equivalent terms for an array of coil elements (i.e., an array having a plurality of antennae). To clarify the distinction from an inductor (i.e., an inductor which typically includes a coil of wire for lower frequencies, and which is sometimes simply called a coil) that is not being used as an antenna, such inductors will be called herein “inductor coils” or simply “inductors”. In some embodiments, the RF-coil device is an MR-RF-coil device that is configured to be used in a high-field MR machine (i.e., it is made of non-ferrous materials and is otherwise compatible with the magnetic and RF fields typically found in such machines) and forms an essential part of the MR machine (such as an MRI machine used to obtain images of structures inside the human body).
In conventional MR machines, there is less concern with compatibility of the high-power RF amplifiers to a high-magnetic-field environment because in conventional MR machines the high-power RF amplifiers are located in a control room, and are at a distance from the remote antenna array located in the magnet bore next to the patient.
There is a long-felt need for a more efficient and flexible way to obtain and connect high-power RF signals to one or more antenna arrays in an MR machine. This need also applies to other high-power RF-transmit-antennae signals.